Since WordPress 3.9 the ability to add padding around an image without manually editing the images HTML directly, or adding a CSS Class to an image has been removed.
If you would like to add the ability to configure a border and margins around your images through the Advanced Options of your image you can download the Advanced Image Styles
plugin. Once the plugin has been downloaded to your local computer follow our article on How to Install a WordPress Plugin
to install it on your site.
Once installed, the settings shown below to add an image border and margins will be available in your Advanced Options screen.
Note: Margins are in pixels.